The present invention relates to electromagnetic coils, and more particularly to multilayer printed circuit board electromagnetic coils.
Electromagnetic coils are used in a wide variety of electrical applications in connection with the inductive transfer of power. For example, different forms of electrical coils are used in transformers, inductive power couplings and motors. Historically, electrical coils have been formed by wrapping a strand of wire into one or more loops. Typically, the diameter of the coil, the type and diameter of the wire, the number of loops (or turns) and other characteristics of the wire and the coil are selected to provide the desired electromagnetic power transfer characteristics.
It is well known that alternating electrical current (AC) has a tendency to distribute itself within a conductor so that the current density near the surface of the conductor is greater than at its core. This phenomenon is commonly referred to as the “skin effect.” The skin effect causes the effective resistance of a conductor to increase with the frequency of the AC current. In an effort to overcome the skin effect, electromagnetic coils used in high frequency applications are often wound from litz wire. Litz wire can be generally characterized as a special type of wire that includes many thin wires, individually coated with an insulating film and twisted together. The individual wires are combined and twisted following a carefully prescribed pattern often involving several levels of twisting (groups of twisted wires are twisted together, etc.). Typically, the wire will be twisted so that each individual strand spends a substantially equal amount of time in proximity to the paired coil. Accordingly, each strand intercepts a substantially equal amount of magnetic flux lines from the paired coil and contributes substantially equally to the self or mutual inductance characteristics of the coil. Because of the combination of separate smaller wires, the combined conductor has greater surface area than a solid conductor using the total cross sectional area and thereby has reduced skin effect. As a result of this and the unique twisting configuration, the power losses associated with litz wire coils can be substantially lower than conventional solid wire coils when used in high-frequency applications. Even with its advantages, litz wire suffers from a number of disadvantages. First, the resistance of a litz wire coil is higher then theoretically achievable because individual strands are round and coated with insulator so that the overall cross-section includes a substantial amount of non-conducting elements, such as air and insulator. Second, the resulting structure is relatively delicate and each strand is subject to breakage. An outer sheath is often incorporated in an attempt to protect the strands. This sheath adds to the overall cost and provides even more resistance over that theoretically achievable. Third, the conductors are thermally insulated and have no heat-carrying path aside from the conductors themselves. So, power handling can be reduced because of thermal considerations. Fourth, the manufacturing process for litz wire and litz wire coils is relatively expensive and requires special, costly equipment. Fifth, the litz wire may be bulkier than desired for some applications because of packing density from wire to wire and the space occupied by the insulation between strands.
Wire coils are relatively expensive to manufacture (particularly litz wire coils), occupy a relatively large amount of space and often require mechanical mounting of the coil to a printed circuit board. To address these issues, it is known to integrate a coil directly into a printed circuit board, for example, by forming the coil on the circuit board using a spiral-shaped trace. In some applications the printed circuit board includes multiple layers of spiral traces that are joined together by vias to form a coil of the desired number of turns (e.g. U.S. Pat. No. 6,914,508 to Ferencz et al, which issued on Jul. 5, 2005). Although printed circuit board coils can present some advantages over wire coils, conventional printed circuit board coils suffer from certain problems faced by conventional solid wires, such as those associated with uneven distribution of induced current and uneven distribution of inductance within the PCB coil. Further, stacked PCB coils can introduce unwanted parasitic capacitance due to some of the coils receiving more of the magnetic field than others. Ultimately, this can result in higher resistance and losses.